B-meson

A B-meson is a type of meson so-called because
it's composed of one bottom (b) antiquark and one up, down, strange, or
charm quark. Its antiparticle,
the B-bar meson, consists of a bottom quark and an up, down, strange, or
charm antiquark. The B meson is a heavy particle; its mass of 5.28 GeV/c2 (due mostly the high mass of the bottom quark that it contains) is five
times greater than that of the proton.

B mesons exhibit CP violation, which
makes them ideal particles for studying the tiny differences between matter
and antimatter and understanding why
there is such a preponderance of matter over antimatter in the present-day
universe.